In engines mounted in leisure vehicles such as motorcycles, oil reserved in an oil pan provided at a bottom portion of the engine or in an oil tank externally and separately mounted thereon is suctioned up by an oil pump driven in synchronization with an engine speed of the engine and is delivered through oil passages provided within the engine to lubricate and cool a crankshaft, a camshaft, or a transmission. In recent years, some engines have been equipped with a hydraulically-powered variable valve timing system configured to change a phase angle of the camshaft with respect to the crankshaft or a hydraulically-powered tensioner of a camshaft drive system of the engine. In such engines, the oil is fed to the system or the component to drive them (see e.g., Japanese Laid-Open Patent Application Publication No. Hei. 7-127661).
To be specific, the oil is output from the oil pump and is filtered by an oil filter. Thereafter, the oil is cooled to a specified temperature in an oil cooler. The resulting oil is fed to engine components to lubricate them and is delivered through a branched oil passage to the hydraulically-powered variable valve timing system, etc. to drive it.
In a conventional engine, the oil output from the oil pump generates a pressure loss in the oil cooler, the oil filter, etc., which is likely to be highly resistant to the oil flowing therein, and is then fed to a hydraulically-powered device such as the hydraulically-powered variable valve timing system. This causes a loss in the oil pressure for driving the hydraulically-powered device. Because the oil to be fed to the hydraulically-powered device is required to have a suitable oil pressure even when the engine is running at a low speed and thereby the oil output from the oil pump becomes less, it is desirable to minimize the pressure loss generated in the oil output from the oil pump. If a large-volume oil pump is equipped in the engine to obtain a sufficient oil pressure, then the size and cost of the engine increase. If the rotations of the oil pump increase in number, then pumping losses increase, causing reduced engine output efficiency.
In order to supply sufficient oil pressure to the hydraulically-powered device, an oil pump exclusively used for driving the hydraulically-powered device may be provided separately from the oil pump for lubrication. In that case, the number of oil pumps undesirably increases, leading to an increase in an installation space and cost.